Simply lumping all mass killings together or even all mass shootings together will inherently obscure any systematic trends because it averages over several completely different types of events, where the methods, psychology, and motives of the killings have little in common.
Define any concept broadly enough (and thus make it useless) and you will find little systematic variance in it. Just consider the extreme example of "There is no less STUFF happening now than in the past."
A person who mows down 20 strangers with the primary goal of mass murder is engaged in a completely different type of act than aperson who kills his wife and her lover in a jealous rage, or happens to kill two people during a robbery, or when 3 gang members shoot 2 rival gang members in a drive by. All of those events are treated as though they are identical events by all the analyses in the OP. Yet only the first event is what most people think "mass shootings" refer to, and is the only event where a largely indiscriminate "mass" of people are shot at for the purpose of killing them rather than in the context of another crime. Should OJ be analyzed as though his actions were the same as the Orlando shooter or the Newton, CT shooter?
When stricter (thus more meaningful and valid) criteria are used to categorize events as similar, trends emerge and show an increase in mass shootings over time.
Mother Jones used critieria related to the first type of incident, where those shot at were not specifically selected, it occured in a "public place" (didn't occur in a private residence), and at least 3 people were killed. There is a strong trend of more such events in the last decade. There are more in the last 5 years than prior 5 years, which in turn had more than the 5 years prior to that. In the last 10 years, 8 of them had 3 or more such events. In the 23 years before that, there were only 4 years with 3 or more such events.
The FBI data show similar increasing trends in "active shooter" situations, which uses criteria with have high overlap with the Mother Jones criteria and include Newton, Orlando, and San Bernadino, but would not include OJ simpson, most gang wars.
Note that the OP link implicitly shows a similar trend when it acknowledges that the number of deaths per event is increasing. That is because in real mass shootings of the type above there are more deaths than the majority of gang shootings or crimes of passion with more than one victim that the OP analyses wrongly categorize as "mass shootings" (which qualify in only the most technical but meaningless sense of the term).
